For soft x-ray or EUV projection lithography there is a need for a high repetition-rate laser plasma source that can radiate at a specific wavelengths in the EUV part of the spectrum and capable of operating at approximately 1200 Hz. The wavelength of the EUV radiation must correspond to the peak reflectivity of the precision multilayer coated reflective optics utilized in this approach. At the present time the wavelengths of choice are 13 nm and 11.7 nm, but in the future other wavelengths may be preferable. This type of source should comprise a compact high repetition-rate laser and a renewable target system capable of operating for prolonged periods of time. For a production line facility, there would need to be uninterrupted system operation for periods of approximately 3 months. That is, uninterrupted operation for some 109 shots. To make the irradiation system cost effective, current system cost scenarios indicate the unit shot material cost must be in the vicinity of ?10xe2x88x926 per shot. Target sources for laser plasma soft-x-ray projection lithography have encompassed several systems such as tape driven targets and solid targets and frozen gases which all have inherent problems.
In a December 1993 publication, M. Richardson, W. T. Silfvast, H. A. Bender, A. Hanzo, V. P. Yanovsky, F. Jin, J. Thorpe, xe2x80x9cCharacterization and control of laser plasma flux parameters for soft-x-ray projection lithographyxe2x80x9d, Appl. Optics 32, 6901-6910 (1993), there is included a characterization of plasma particulate emissions and suggestions for reducing the fluxes impact on optical components including: use of a background gas of helium (page 6907, first column); use of spinning apertures (page 6908, first column); use of pulsed gas jets to deflect the particles (page 6908); use of magnetic deflection of the particles after charged with an high-current electronic beam (page 6908); and, laser light ablation of particles to deflect the particles (page 6908). None of these approaches have been found useful in the 17 or more years since that time.
In U.S. Pat. No. 5,151,928 to Hirose discloses a system, which uses film type tapes as a target source. However, tape driven targets are limited mass targets that are difficult to construct, prone to breakage and produce low velocity debris that can damage the other components such as the mirrors in the laser output system. Furthermore, using and replacing the tapes is generally cumbersome and costly.
Solid targets also have many problems. Known solid target sources include rotating solid wheels of Sn or tin, or copper or gold or other materials. These sources inherently can produce various ballistic particles types that can emanate from the plasma in various directions. When using tin as the target the interaction of the laser light with the plasma, causes highly stripped, energetic ions to be formed by collision ionization. These ions can stream ballistically from the plasma region with velocities of between 106 and 107 centimeters per second. Furthermore, after the interaction of the plasma and the tin target, hot clumps or clusters of target material are boiled off of the target. These materials have been determined to be another form of debris particles that can have velocities in the range of 200 to 2500 centimeters per second. The sizes of these particles range from less than 1 to greater than 200 micrometers. These larger particles can cause cold particle cratering on many of the components used to construct visible and x-ray optical components such as the mirrors and are capable of puncturing thin-fill x-ray filters. All of these debris size particles can degrade the surface quality or the operating performance by over coating the optical elements in the laser output system. Likewise, other solid targets besides tin can produce similar debris type particles.
In summary, since the development of Extreme Ultraviolet Lithography (EUVL) is the leading technology to replace optical lithography, there is a common requirement that there be no collateral particulate emission from the plasma. This leads to the liquid droplet source disclosed in U.S. Pat. Nos. 5,459,771 and 5,577,091 as a bright source in the soft x-rays and EUV radiation of 13 nm and 11.6 nm radiation. Unfortunately, it has been found that with extended life ( greater than 107 shots), ions from this plasma still sputter or ablate the surface layers of the multi-layer EUV lithography optical systems.
The first objective of the present invention is to provide an inexpensive, sequencing target system as a laser plasma x-ray source having an increased operational lifetime.
The second object of this invention is to provide a target system that prevents ions hitting the collector mirror or associated components of a lithographic system by the use of a repeller electric field structure.
The third object of this invention is to provide a laser plasma target source that can operate at a frequency of approximately 1000 Hz to 100 kHz for an extended operational lifetime.
The fourth object of this invention is to provide a laser plasma target source for lithographic applications that can operate continuously for an extended operational period of more than 107 shots, and extending beyond that currently needed for an operating lithographic tool.
In a preferred embodiment, a pulse based EUV light source based on laser plasmas produced from a train of microscopic water droplets comprising: an EUV light source; a first collector EUV mirror; and an auxiliary electrode system, preferably a dc potential imposed on said mirror, which provides a repeller field that slows down and reverses the trajectory of ions from said source before they impact the collection mirror.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated schematically in the accompanying drawings and illustrated comparative data showing the surprising operational lifetime of the embodiment and the method employed therein.